In oil wells, natural gas wells, and the like (hereinafter also collectively referred to as “oil wells”), steel pipes referred to as oil country tubular goods (OCTG) such as casings and tubings are used for extraction of underground resources. The steel pipes are sequentially connected to each other, and threaded joints are used for the connection.
Threaded joints for steel pipes are classified into two types: coupling-type joints and integral-type joints. A coupling-type threaded joint is constituted by a pair of tubular goods that are to be connected to each other, of which one is a steel pipe and the other is a coupling. In this case, the steel pipe includes male threaded portions formed on the outer peripheries at both ends thereof, and the coupling includes female threaded portions formed on the inner peripheries at both ends thereof. Thus, the steel pipe and the coupling are connected to each other. An integral-type threaded joint is constituted by a pair of steel pipes as tubular goods that are to be connected to each other, without a separate coupling being used. In this case, each steel pipe includes a male threaded portion formed on the outer periphery at one end thereof and a female threaded portion formed on the inner periphery at the other end thereof. Thus, the one steel pipe and the other steel pipe are connected to each other.
In general, the joint portion at the tubular end where a male threaded portion is disposed is referred to as a pin because it includes an element that is inserted into a female threaded portion. On the other hand, the joint portion at the tubular end where a female threaded portion is disposed is referred to as a box because it includes an element that receives a male threaded portion. Pins and boxes both have a tubular shape because they are constituted by end portions of tubular goods.
Threaded joints for steel pipes are configured such that the male threaded portion of the pin is screwed onto the female threaded portion of the box and accordingly the male threaded portion and the female threaded portion, each being a tapered threaded portion, engage in intimate contact with each other. Basically, this thread seal produced by the engagement and intimate contact of the male threaded portion and the female threaded portion ensures the sealing performance of threaded joints.
In recent years, some drilling techniques such as horizontal drilling and directional drilling have been increasingly employed in oil wells, so that threaded joints are more likely to be subjected to high torque loads. For this reason, there is an increasing demand for threaded joints capable of providing high torque resistance performance. Furthermore, oil well environments in which the above-mentioned drilling techniques are applied are harsh environments with high temperatures and high pressures. To address such harsh environments, threaded joints need to be capable of providing not only high torque resistance performance but also high sealing performance.
In general, coupling-type threaded joints are widely used because they exhibit better sealing performance than integral-type threaded joints.
An example of threaded joints capable of providing high torque resistance performance is a threaded joint employing tapered threads of the dovetail type also referred to as wedge threads. Threaded joints employing wedge threads are configured as follows. The thread width of the male threaded portion gradually decreases along the thread helix in the right-hand screw direction, and the groove width of the corresponding female threaded portion also gradually decreases along the thread helix in the right-hand screw direction. Both the load flanks and the stabbing flanks have negative flank angles, and at the completion of fastening thread, the stabbing flanks are in contact with each other and the load flanks are in contact with each other, whereby the threaded portions as a whole firmly engage with each other. Furthermore, in the fastened state, crests and roots of the threaded portions are in intimate contact with each other. Thus, threaded joints employing wedge threads are capable of providing high torque resistance performance while ensuring sealing performance.
However, in the case of the thread seal using wedge threads, a sufficient seal cannot be formed unless the male threaded portion and the female threaded portion engage in intimate contact with each other by strictly setting manufacturing tolerances such as the radius of curvature for each of the threaded portions. Furthermore, the thread seal can be formed substantially only in the region of complete threads, and therefore, if a threaded joint is designed to have a relatively long incomplete thread region, sealing performance as desired may not be obtained. In particular, when an internal pressure or external pressure is excessively applied, the highly pressurized fluid may penetrate into gaps at the thread seal, thereby incurring the risk of leakage.
As described above, there is a limit to sealing performance that can be ensured solely by the thread seal. For this reason, attempts have hitherto been made to ensure sealing performance against internal and external pressures by providing an internal seal and an external seal separately from the thread seal. The internal seal is formed by contact between a sealing surface of the pin disposed on a free end region thereof, forward of the male threaded portion of the pin, and a sealing surface of the box corresponding thereto. The external seal is formed by contact between a sealing surface of the pin disposed on a region rearward of the male threaded portion thereof, and a sealing surface of the box corresponding thereto.
For example, United States Patent Application Publication No. 2010/0181763 (Patent Literature 1) and International Publication No. WO2004/106797 (Patent Literature 2) each disclose a threaded joint having an internal seal and an external seal in addition to a thread seal. Of these threaded joints of the patent literatures, the threaded joint of Patent Literature 2 is configured such that, in a fastened state, clearances are provided between the crests and the roots of the threaded portions. These clearances allow an excess of the lubricant (hereinafter also referred to as “dope”), applied for fastening thread, to accumulate therein, thereby contributing to prevention of an abnormal increase in the dope pressure.
It is true that the conventional threaded joints disclosed in Patent Literatures 1 and 2 exhibit improved sealing performance against pressures. However, threaded joints having an external seal have a reduced cross sectional area of the critical section of the pin (the cross section in the rearmost end region of the male threaded portion, i.e., the cross section in the male threaded portion closest to the tubular body), and therefore have a reduced resistance to tensile forces. For this reason, it is not desirable to provide an external seal from the standpoint of the threaded joint strength.
In the case of threaded joints having an internal seal, when an excessively high external pressure has been applied thereto, plastic deformation occurs in the region of the sealing surface of the pin in the free end region thereof because the region of the sealing surface does not have a sufficient wall thickness. As a result, there is a risk that sealing performance may not be exhibited.